diff --git a/.github/workflows/docs-preview-deploy.yml b/.github/workflows/docs-preview-deploy.yml
index 39331502..d4b41c4e 100644
--- a/.github/workflows/docs-preview-deploy.yml
+++ b/.github/workflows/docs-preview-deploy.yml
@@ -98,6 +98,7 @@ jobs:
                 'requirements-docs.txt',
                 'scripts/build-docs.sh',
                 'scripts/render-dev-notes.py',
+                'tests/test_docs_404.py',
                 'tests/test_render_dev_notes.py',
                 'zensical.toml',
               ]);
diff --git a/.github/workflows/docs-preview.yml b/.github/workflows/docs-preview.yml
index a6915ff9..2a14e41e 100644
--- a/.github/workflows/docs-preview.yml
+++ b/.github/workflows/docs-preview.yml
@@ -46,6 +46,7 @@ jobs:
               'requirements-docs.txt',
               'scripts/build-docs.sh',
               'scripts/render-dev-notes.py',
+              'tests/test_docs_404.py',
               'tests/test_render_dev_notes.py',
               'zensical.toml',
             ]);
diff --git a/docs/development/index.md b/docs/development/index.md
index db4c4f51..37985f8f 100644
--- a/docs/development/index.md
+++ b/docs/development/index.md
@@ -58,7 +58,9 @@ Verify branded surfaces in both the `default` and `slate` palette schemes. Prefe
 CSS variables in `docs/stylesheets/dev-notes.css` over one-off hard-coded colors.
 Keep asset paths relative to `docs_dir`. Theme templates live in `overrides/`;
 keep the custom `404.html` useful for ordinary missing pages as well as expired
-pull request previews.
+pull request previews. Keep that fallback self-contained: GitHub Pages serves it
+for arbitrary paths where root-relative theme assets do not resolve beneath the
+project site prefix.
 
 ## Validate and preview
 
diff --git a/overrides/404.html b/overrides/404.html
index a2a87005..0ce3477a 100644
--- a/overrides/404.html
+++ b/overrides/404.html
@@ -1,28 +1,130 @@
-{% extends "main.html" %}
-
-{% block htmltitle %}
-  <title>Page not found - {{ config.site_name }}</title>
-{% endblock %}
-
-{% block content %}
-  <h1>Page not found</h1>
-  <p id="not-found-message">The requested documentation page does not exist.</p>
-  <p>
-    <a id="not-found-home" href="{{ config.extra.homepage | d(nav.homepage.url, true) | url }}">
-      Return to OpenShell Research
-    </a>
-  </p>
-  <script>
-    (() => {
-      const previewPath = window.location.pathname.match(
-        /^(.*\/)pr-preview\/pr-[1-9][0-9]*(?:\/|$)/,
-      );
-      if (previewPath) {
-        document.title = `Preview unavailable - {{ config.site_name }}`;
-        document.getElementById("not-found-message").textContent =
-          "This pull request preview has expired or was removed.";
-        document.getElementById("not-found-home").href = previewPath[1];
-      }
-    })();
-  </script>
-{% endblock %}
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1">
+    <meta name="robots" content="noindex">
+    <meta name="color-scheme" content="light dark">
+    <title>Page not found - {{ config.site_name }}</title>
+    <style>
+      :root {
+        color-scheme: light dark;
+        --accent: #5a8f00;
+        --background: #f6f7f8;
+        --border: #d9dde3;
+        --button-text: #111315;
+        --card: #ffffff;
+        --muted: #52606d;
+        --text: #171a1f;
+      }
+
+      @media (prefers-color-scheme: dark) {
+        :root {
+          --accent: #76b900;
+          --background: #111315;
+          --border: #343a40;
+          --card: #1b1f23;
+          --muted: #b6bec8;
+          --text: #f4f6f8;
+        }
+      }
+
+      * {
+        box-sizing: border-box;
+      }
+
+      body {
+        align-items: center;
+        background: var(--background);
+        color: var(--text);
+        display: flex;
+        font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont,
+          "Segoe UI", sans-serif;
+        justify-content: center;
+        margin: 0;
+        min-height: 100vh;
+        padding: 2rem;
+      }
+
+      main {
+        background: var(--card);
+        border: 1px solid var(--border);
+        border-radius: 1rem;
+        box-shadow: 0 1rem 3rem rgb(0 0 0 / 12%);
+        max-width: 38rem;
+        padding: clamp(2rem, 7vw, 4rem);
+        text-align: center;
+        width: 100%;
+      }
+
+      svg {
+        display: block;
+        height: 5rem;
+        margin: 0 auto 1.5rem;
+        width: 5rem;
+      }
+
+      h1 {
+        font-size: clamp(2rem, 8vw, 3.5rem);
+        letter-spacing: -0.04em;
+        line-height: 1;
+        margin: 0 0 1rem;
+      }
+
+      p {
+        color: var(--muted);
+        font-size: 1.05rem;
+        line-height: 1.6;
+        margin: 0 auto 2rem;
+        max-width: 30rem;
+      }
+
+      a {
+        background: var(--accent);
+        border-radius: 0.5rem;
+        color: var(--button-text);
+        display: inline-block;
+        font-weight: 700;
+        padding: 0.8rem 1.15rem;
+        text-decoration: none;
+      }
+
+      a:hover {
+        filter: brightness(0.9);
+      }
+
+      a:focus-visible {
+        outline: 3px solid var(--accent);
+        outline-offset: 4px;
+      }
+    </style>
+  </head>
+  <body>
+    <main aria-labelledby="not-found-title">
+      <svg viewBox="0 0 100 100" role="img" aria-label="OpenShell Research">
+        <path d="M50 9 84 23v28c0 21-15 35-34 41-19-6-34-20-34-41V23Z" fill="#76b900"/>
+        <g fill="none" stroke="#fff" stroke-width="6" stroke-linecap="round" stroke-linejoin="round">
+          <path d="m37 41 12 9-12 9"/>
+          <path d="M55 59h12"/>
+        </g>
+      </svg>
+      <h1 id="not-found-title">Page not found</h1>
+      <p id="not-found-message">The requested documentation page does not exist.</p>
+      <a id="not-found-home" href="/OpenShell-Research/">Return to OpenShell Research</a>
+    </main>
+    <script>
+      (() => {
+        const previewPath = window.location.pathname.match(
+          /^(.*\/)pr-preview\/pr-[1-9][0-9]*(?:\/|$)/,
+        );
+        if (previewPath) {
+          document.title = "Preview unavailable - {{ config.site_name }}";
+          document.getElementById("not-found-title").textContent = "Preview unavailable";
+          document.getElementById("not-found-message").textContent =
+            "This pull request preview has expired or was removed.";
+          document.getElementById("not-found-home").href = previewPath[1];
+        }
+      })();
+    </script>
+  </body>
+</html>
diff --git a/scripts/build-docs.sh b/scripts/build-docs.sh
index e264cd3b..41c9292e 100755
--- a/scripts/build-docs.sh
+++ b/scripts/build-docs.sh
@@ -32,3 +32,4 @@ python -m pip install -r requirements-docs.txt
 
 python scripts/render-dev-notes.py
 zensical build --clean --strict
+REQUIRE_RENDERED_404=1 python tests/test_docs_404.py
diff --git a/tests/test_docs_404.py b/tests/test_docs_404.py
new file mode 100644
index 00000000..0ad647b5
--- /dev/null
+++ b/tests/test_docs_404.py
@@ -0,0 +1,51 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+import os
+from pathlib import Path
+import re
+import unittest
+
+
+ROOT = Path(__file__).resolve().parents[1]
+TEMPLATE = ROOT / "overrides" / "404.html"
+RENDERED = ROOT / "site" / "404.html"
+
+
+class Docs404Tests(unittest.TestCase):
+    def test_template_is_standalone_and_self_contained(self) -> None:
+        html = TEMPLATE.read_text(encoding="utf-8")
+
+        self.assertTrue(html.lstrip().lower().startswith("<!doctype html>"))
+        self.assertNotIn("{% extends", html)
+        self.assertNotRegex(html, r"<(?:img|script)[^>]+src=")
+        self.assertNotRegex(html, r"<link[^>]+rel=[\"']stylesheet")
+        self.assertNotRegex(html, r"https?://")
+        self.assertIn("<style>", html)
+        self.assertIn("<main aria-labelledby=", html)
+        self.assertIn("prefers-color-scheme: dark", html)
+
+    def test_template_handles_expired_preview_paths(self) -> None:
+        html = TEMPLATE.read_text(encoding="utf-8")
+
+        self.assertIn("pr-preview", html)
+        self.assertIn("Preview unavailable", html)
+        self.assertIn("This pull request preview has expired or was removed.", html)
+
+    def test_rendered_page_has_no_root_asset_dependencies(self) -> None:
+        if os.environ.get("REQUIRE_RENDERED_404") != "1":
+            self.skipTest("rendered output is checked after the documentation build")
+        if not RENDERED.exists():
+            self.fail("site/404.html was not generated")
+
+        html = RENDERED.read_text(encoding="utf-8")
+        asset_reference = re.compile(
+            r"(?:href|src)=[\"']/(?:assets|stylesheets|javascripts)/",
+        )
+        self.assertNotRegex(html, asset_reference)
+        self.assertIn("<style>", html)
+        self.assertIn("This pull request preview has expired or was removed.", html)
+
+
+if __name__ == "__main__":
+    unittest.main()
